Hard service in Iraq is wearing out some of the US military's core weapons. Tanks, armored vehicles, and aircraft are being run at rates two to six times greater than in peacetime, Secretary of Defense Donald Rumsfeld told Congress earlier this month.

The bad news here is they may need to be replaced. But there's good news too, according to Secretary Rumsfeld: It's possible they can be replaced with something better.

The need to refurbish equipment "is providing an opportunity to adjust the capabilities of the force earlier than otherwise might have been the case," Rumsfeld told the House Armed Services Committee on March 10.

Perhaps the same might be said of the military as a whole. Two years after the invasion of Iraq, the tough work of helping rebuild a nation while fighting an insurgency has profoundly affected the organization and deployment of United States forces. Whatever Iraq becomes, the American way of war may never be the same.

Throughout the services there's a new emphasis on mobility, guerrilla-fighting skills, and special forces. These changes might have occurred whether President Bush ordered the toppling of Saddam Hussein or not. But the urgency created by war may be making it easier for Secretary Rumsfeld to pursue a long-sought transformation of the Pentagon.

"I see not so much a direct response as an accelerated implementation of a plan Bush advisers already had in place," says Loren Thompson, a defense expert at the Lexington Institute.

A look at what happened in Iraq

On one level the effect of Iraq on the structure of the US armed forces is clear, and saddening. Over 1,500 Americans have been killed, and thousands more wounded, by the fighting.

US units stormed over the border from Kuwait prepared to fight conventional battles, and that aspect of the war they handily won. It took some time for commanders in the field and in Washington to realize that in fact their mission had not been accomplished. The depth and ferocity of the insurgent resistance took many by surprise - as a shortage of armor for vehicles showed.

It's now a Washington truism that the biggest mistake made in the Iraqi operation was the lack of preparation for stability operations in the immediate aftermath of the fall of Saddam. This may have involved more than a paucity of troops to police the streets, or unpreparedness for the dangers of roadside bombs.

Dov Zakhein, Pentagon comptroller during the first Bush administration, complained at a recent Washington seminar that at first during the occupation the military financing system didn't work. He could start money moving in Washington, but it wouldn't get to Iraq, or wouldn't get to the right place.

"It's obvious Iraq has hugely taxed the US military," says Michael O'Hanlon, a military expert at the Brookings Institution.

Protection and recruitment

Over the past two years US commanders have scrambled to remedy this situation. The Army and Marines have created the kind of units needed for counterinsurgency. The Pentagon is increasing the size of special forces - 500 new Green Berets are scheduled to be added this year, for instance. Veteran special-forces operators are now eligible for reenlistment bonuses of up to $150,000.

Slowly, after some missteps, the military is moving to provide troops with extra protection tailored to the manner in which insurgents fight. This means body armor, and armor for Humvees and other transport vehicles. But at a recent hearing members of Congress still pleaded with Gen. John Abizaid, commander of US Central Command, to ship a stockpile of ballistic glass to Iraq, so that troops could custom-fit window shields.

General Abizaid admitted that there was a lesson for the military in the fact that troops were unprepared to combat improvised explosives and other guerrilla weapons.

"We have to design our armed forces for the 360-degree battlefield and not the linear battlefield," he told House Armed Services Committee members.

And deployments - plus tough fighting - are clearly affecting the military's ability to attract and retain troops. Prior to September 11, 2001, the Department of Defense had about 250,000 uniformed personnel stationed overseas at any one time. Now that figure is 400,000.

Polls show that many potential enlistees, and their parents, have become concerned about the possibility of ending up in an Iraqi firefight. Last month the Army missed its target of 7,050 new active duty recruits by almost 2,000.

Yet Iraq - and more broadly, the fighting in Iraq and Afghanistan - is also providing the military with opportunity for comprehensive change. The US may have gone to war with the Army it had, to paraphrase Secretary Rumsfeld. But it's likely to leave the war with armed services that are considerably different.

Take the Navy. In the old days it had a strict 18-month cycle for ship deployments, notes Thompson of the Lexington Institute. This meant six months at sea, followed by six months downtime, and six months spent preparing for the next deployment.

That's been changed so that deployments are less automatic, and more responsive to events in the world. Such tactics as switching crews while ships remain at sea in effect increases the Navy's size, as it can lower the number in port.

The Navy "now has a completely new model based on surging in response to threats," says Thompson.

The Air Force, for its part, is inevitably becoming less fighter-centric. The most important airplane in Iraq, according to Abizaid of Central Command, has been the C-17 airlifter. For this reason, plus budget pressure, the projected numbers of the new F-22 fighter are dwindling.

Then there's the Army. With the Marines, it has shouldered most of the Iraqi fighting, and suffered many of the casualties.

Iraq has given the Army an opportunity to test and change its new Stryker brigades, which, with their wheeled armored vehicles, are intended as a lighter and faster-acting fighting force. It has put the service on notice of its need for greater modularity - in which each division might be less unique, with interchangeable smaller units.

"The Army has been through very tough times in the last four years. What has come out is a determination to really completely change its organization," says Mr. Thompson.

Right now the US military is embarking on a new Quadrennial Defense Review (QDR) process, a periodic exercise in which the Pentagon leadership sets down a broad vision for the structure and use of US forces in the world.

If nothing else the QDR this time may allow the Pentagon and the Congress to draw on the Iraq experience and decide how to balance the demands of peacekeeping and war-fighting in the modern age.

"We need as a nation to decide what we want our military to be," says Jack Spencer, a military analyst at the Heritage Foundation.